wifi channel troubleshooting c6900

Automatic channel selection is usually a good bet.  The router is detecting how much spectral power is present on a given channel, and it's picking the one with the least power so it has the best chance of being heard by the surrounding devices.  It's kind of like picking the quietest room if you want to have a conversation.

The 2.4 GHz band nominally has channels 1-11, but these channels actually overlap somewhat.  1, 6, and 11 are far enough apart that there's not a whole lot of overlap.  The same argument can be made for any adjacent channels in the 5 GHz band.  Even though they have a defined frequency range, the power doesn't completely cut off at the edge of that band; it's just low enough to fall below some threshold set in the specification.

In addition to channel selection, the WiFi signals are also modulated (i.e. coded) such that traffic on two different networks operating on the same band can be distingished.  It only gets problematic when they're trying to transmit right next to each other at exactly the same time... but this is not often an issue unless you're living in a packed high-rise apartment building in a big city.  I can see ~30 of my neighbors' networks at any given time and don't have connectivity issues.

Also, 5 GHz channels tend to do better in congested areas, and I don't think you need to feel tied to channel 153.  The eight standard channel options avaialble on my router (in the US) are all identical according to the channel specifications listed here:

https://en.wikipedia.org/wiki/List_of_WLAN_channels#5_GHz_(802.11a/h/j/n/ac)

In general, 5 GHz channels have better rejection (better isolation from neighboring channels) than their 2.4 GHz counterparts.

As far as the logic of picking a channel, if you don't think the channel you're using is providing the best performance, and automatic selection isn't doing it for you, try changing it up and see if it gets better.

Another thing that may help is making sure your router isn't sitting on the floor.  Elevating it just a foot or two will help considerably.  Also, make sure it's not laying on its side.  If it's meant to stand up, the antennas are probably oriented along its height, and won't perform as well they're sideways.  The radiation pattern of dipole antennas (the kind of antennas in most routers) is kind of a flat disc/donut shape, and you want to be in roughly the same plane as that disc.  A picture of what I'm describing poorly, where your antenna would be along the z-axis:

http://www.antenna-theory.com/antennas/norm3D1lam.jpg

This is all getting off on a tangent... just another thing to check if you're having trouble with a weak signal getting swallowed up by your neighbors.

Good luck!